Because of the enhanced strength and weight savings possible with composite materials as compared with metals, the aerospace industry is seeking to incorporate such composite materials in aerospace structures. These composite materials incorporate high strength, high elasticity modulus fibers embedded in a plastic matrix to form the structure. A number of these materials are available such as graphite-epoxy composites, boron-epoxy composites, kevlar-epoxy composites, hybrid composites and thermoplastic composites. Because the primary load carrying components of these composites are the fibers, these composites are very strong lengthwise and cross-wise (when cross plys and/or diagonal plys of fibers are used), but relatively weak through their thicknesses since the strength in this direction is no more than the strength of the cured plastic resin of the matrix. One of the primary problems encountered in using such composite materials in such aerospace structures is the difficulty in producing satisfactory fastener joints required to join a plurality of pieces of this composite material together and/or composites to metal structural members. Satisfactory fastener joints in these composite materials require that the joint carry the load for which the joint is designed and also have a long fatigue life.
The techniques such as interference fit and coldworking normally used to enhance the load carrying strength and the fatigue life of fastener joints in all metal structures have, by and large, had the reverse effect on fastener joints in composite materials. This is because such techniques require that the holes through the work pieces be deformed to conform to the fastener or to place a compressive stress in the material of the work pieces about the holes while this deformation of the holes in metal work pieces generally does not affect the strength of the work piece, deformation of the holes in work pieces of composite materials usually causes local delamination of the composite material around the hole as well as crazing or crushing of the plastic matrix of the composite material around the hole. When this occurs, the fatigue life of the composite material is reduced rather than enhanced, thereby making the joint subject to early failure. To insure that no deformation of the composite material of the work pieces about the holes occurs, fasteners joining these work pieces have been installed in clearance fits so that no bearing contact occurs between the bearing section on the shank of the fastener and the work pieces. This has made it difficult to produce fastener joints in these composite materials which transmit the desired design load through the joint while at the same time producing the desired fatigue life in the joint.
Another problem associated with the use of the fastener joints in composite materials, especially those composite materials containing graphite, is galvanic corrosion. While the graphite itself, being at the extreme cathodic end of the galvanic series, is highly resistant to galvanic corrosion, almost all metal fasteners used in the fastener joints are corroded relatively fast. This results in early joint failure.
Yet another problem encountered when using fastener joints in composite materials is that the composite material is frequently damaged if the joint needs to be disassembled and then reassembled for repair. This is also because the composite material is relatively weak through its thickness, and loading of the material axially of the fastener holes tends to deteriorate or destroy the plastic matrix.